Heating method for direct-arc furnace

ABSTRACT

HEATING METHOD FOR DIRECT ARC FURNACES. AT AN INITIAL STAGE OF MELTING THE FURNACE CHARGE, THE TIPS OF THE ELECTRODES ARE SPACED APART BY RELATIVELY LONG DISTANCES. AS THE MELTING IS ADVANCED, THIS SPACING IS GRADUALLY REDUCED, WHILE THE ELECTRODES ARE LOWERED.

United States Patent Show Yasukawa Kanagawa-ken;

Masayuki Aoshika, Saitama-ken, Japan 836,798

June 26, 1969 June 28, 1971 lshikawajima-Harima Jukogyo Kabushiki KaishaTokyo-to, Japan Inventors Appl. No Filed Patented Assignee HEATINGMETHOD FOR DIRECT-ARC FURNACE 3 Claims, 6 Drawing Figs.

U.S.C1 13/34, 13/9 Int. Cl H05b 3/60, HOSb 7/10 [50] Field of Search13/9, 12, 13, 34

[56] References Cited UNITED STATES PATENTS 858,400 7/1907 Kugelgen, eta1. 13/34 3,151,266 9/1964 Hannappel, et a1 13/34X PrimaryExaminer-Bernard Gilheany Assistant Examiner-Roy N. Envall, Jr.AttorneyN01te and Nolte ABSTRACT: Heating method for direct arcfurnaces. At an initial stage of melting the furnace charge, the tips ofthe electrodes are spaced apart by relatively long distances. As themelting is advanced, this spacing is gradually reduced, while theelectrodes are lowered.

PATENTED JUN28 l97| SHEET 1 OF 2 FlGg2 INVENTORS SHOZO-YASUKAW MASAYUKIAOSHI ATTORNEYS PAIENTEDJMN28I97| 8,588,308

SHEET 2 OF 2 FIG. 6

COMPARISON OF LINING DAMAGE COEFFICIENT R AT TOT ARC FURNACE RE= LININGDAMAGE INDEX PA EARC POWER (MW/POLE) ,v ARC VOLTAGE (v) L =THE MINIMUMDISTANCE IN METERS BETWEEN AN ELECTRODE AND THE LINING 5800 DIA. SHELL5IOO i CONVENTIONAL ARC FURNACE l I R: PCD |3?0 T I-6H2 ARC FURNACE OFSLANT 508 ELECTRODE SYSTEM R" 0.289 0.75 PCB 870 I 2 I861 R. 0.385

77% r v/w ATTO RN EYS HEATING METHOD FOR DIRECT-ARC FURNACE BRIEFSUMMARY OF THE INVENTION The present invention relates to a heatingmethod for direct-arc furnace.

The primary object of the present invention is to eliminate the defectsencountered in the conventional three-phase AC arc furnaces such asnonuniform melting, hot spot phenomenon on the furnace wall, refractoryunbalanced three-phase powers and the flicker phenomenon.

In the conventional three-phase AC arc furnaces, the electrodes forgenerating arcs are arranged to vertically move and make electricalcontact with the charge or scrap. When such furnaces are employed insteel production, the following defects are generally observed:

1. The electrodes are spaced apart from each other by a relatively longdistance in order that the material such as scrap metal charged in thefurnace may be uniformly melted. Therefore when the furnace wall isexposed as the melting operation progresses (that is when the flat bathstate is reached), local breakage of the furnace wall refractory (hotspot phenomenon) tends to occur because of the heat radiation from thearcs and the arc flares directed toward the furnace wall. The degree ormagnitude of the damage to the refractory of the furnace wall may beexpressed by the following relation and is increased in inverseproportion to the square of the distance between each of the electrodesand the wall.

P VA

where R refractory damage index (index of heat absorbed by per unit areaof the refractory) P are power (MW/pole) V A arc voltage (V) Y L theminimum distance (in meters) between an electrode and the refractory,and

K coefficient it is very disadvantageous for proper melting to make thedistance between the electrode and the refractory any shorter, in orderto decrease the extent of the hot spot phenomenon. Furthermore, it isreadily seen from Eq. l that the higher the damage to the refractory,the higher the power.

(2) ln the conventional electrode arrangement, the length of the centralelectrode is shorter and is influenced by the flux leakage from the sideelectrodes so that the impedance of the center electrode circuit becomessmaller, thereby bringing about the unbalanced three-phase powers.Therefore, the charge or scrap is not melted uniformly and especiallythe furnace wallrefractory opposite the center electrode is subjected tothe hot spot phenomenon. The circuit impedance is given by the followingrelation:

where Z circuit impedance in ohm R resistance of conductor in ohm or21rf(f= frequency) and L, effective inductance in H. The effectiveimpedance of each phase when the balanced three-phase current flows maybe expressed by the following equations:

where L, to L, effective inductance of each phase (1-!) D to D distancebetween different phase conductors (geometrical average distance betweendifferent phase in meters) R to R extension of one phase of conductors(geometrical average distance, in meters) The length of the centerelectrode or conductor is shorter and has a smaller resistance R. It isfurther influenced by the leakage flux from the other conductors orelectrodes so that its effective inductance L. is smaller. Thus, thecircuit impedance of the center electrode becomes smaller. Theunbalanced effective are power due to unbalanced impedance becomes moreremarkable as the operating current is increased.

There has been known in the art the arc furnace in which the sideelectrodes or conductors are made shorter than the center electrode orconductor and the electrodes are arranged in the form of an invertedtriangle in order to balance the three-phase power and to prevent thehot-spot phenomenon of the furnace wall refractory opposite to thecenter electrode. In this case, the dimensions of the electrode holdersare limited so that the electrodes are in some cases spaced apart fromeach other by a distance longer than the one used in above describedconventional furnace. Therefore, the distance between the electrode andthe furnace wallrefractory is further reduced so that the hot spotphenomenon tends to occur more remarkably. The increased spacing betweenthe electrodes means the increase of the geometrical average distancebetween the different phase electrodes so that the effective impedanceis increased, thereby increasing the furnace circuit impedance. Thus,the voltage drop in the arc generating circuit is increased so that thepower cannot be utilized efficiently. V

In the conventional arc furnaces of the type in which the electrodes arearranged in a conventional manner, the spacing between the electrodescannot be reduced sufficiently because of the limit imposed by thedimensions of the electrode holders even though it is tried to minimizethe hot spot phenomenon by reducing the spacing. This tendency willbecome more remarkable when electrodes having a large diameter are usedin high power operation (HP UHP).

The spacing between the different phase conductors on the side of theprimary winding of the furnace transformer is varied by the variation ofthe height of the electrode supporting arm, but may be regarded asconstant during the whole process of melting and refining so that theimpedance of the circuit on the side of the secondary winding of thefurnace transformer may remain unchanged.

Therefore, there are two problems in furnace operation. That is,

i. Especially in case of the furnace having a lower impedance secondarycircuit, arcs are unstable and not continuous at the initial stage ofmelting so that the power is not utilized to its full extent.Furthermore, large current tends to flow due to the short circuitbetween the electrodes and the scrap metal or the arcs are extinguishedso that variation in voltage of the power source (flicker phenomenon)occurs and presents another problem.

ii. In case of the furnace having a large impedance secondary circuit,the arcs are stable at the initiation of melting and the flickerphenomenon presents no problem. But the power cannot be utilized to itsfull extend even after reaching the stage atwhich the arcs arestabilized.

Generally the following methods are employed in order to prevent theflicker. I

A. On the side of the power source system:

1. The system is changed so that more power may be supplied.

2. Capacitors are connected in series to the power source.

3. Boosters or compensation coupling reactors are interconnected betweenthe bus to the arc furnace and a bus to other loads.

B. Within arc furnace plants:

1. A series reactor is connected to the primary of the furnacetransformer.

2. A saturable reactor is connected in series to the primary of thefurnace transformer.

3. Saturable reactors are connected in parallel with the primary of thefurnace transformer.

4. A buffer reactor is connected between the power source and thefurnace transformer and a synchronous phase modifier is connected inparallel.

However. the cost of providing such remedies often is almost equal tothe cost of installing an arc furnace. For this reason development ofthe arc furnace has been hindered by the indicated problems.

In case of steel production by are furnaces which has been operatedrecently at increased power, the above described problems have still notbeen solved satisfactorily. Therefore, especially in case of high powerarc furnaces, it is extremely difficult to operate them at fullcapacity.

According to the present invention, the electrodes in the three-phase ACarc furnace are obliquely moved so that they are initially spaced apartfrom each other by a relatively longer distance, in the upper space ofthe furnace, and the spacing is gradually reduced as they are lowered.By this operation, the mutual geometrical average distances between theseveral electrodes (constituting the respective phase secondaryconductors) and also between their current supply leads (flexible cablesand buses upon suitable arms) are increased, thereby increasing thecircuit impedance when the tips of the electrodes are still in the upperportion of the furnaces. On the other hand, when the tips of theelectrodes are lowered, their mutual geometrical average distances arereduced, so that the circuit impedance is reduced. Thus, in the initialstage when the tips of the electrodes are still in the upper portion ofthe furnace, the mutual distances between the flexible cables, busesupon the arms and the electrodes are relatively longer, so that the arcsmay be stabilized in a relative high furnace while the flicker can beprevented, thereby uniformly melting the charge. As the meltingoperation progresses, the three mutual distances between electrode cableand bus system are gradually reduced. When electrodes reach the bottomof the furnace and the molten metal bath is formed therein so that theconditions for stabilizing the arcs at the tips of the electrodes aresatisfied, the mutual distances between the electrode, cable and bussystems are minimized. In the flat bath state where the furnace wall isexposed, the low impedance high current-short arcs are concentrated tothe center of the molten metal so that the efficiency of transmission ofheat generated by the arcs to the molten metal may be remarkablyimproved while the hot spot phenomenon of the furnace wall refractorymay be prevented, thereby permitting quick temperature rise.

Thus, it will be understood that a high power three-phase AC arc furnaceusing the present invention can operate at full capacity, thus bringingabout various advantages in the steel production.

The above and other objects, features and advantages of the presentinvention will become more apparent from the following description ofthe illustrative embodiments thereof taken in conjunction with theaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIGS. 1 to 4 are plan views of heatingsystems according to the present invention;

FIG. 5 is a side view illustrating the essential parts of a direct-arcfurnace shown in FIG. I; and

FIG. 6 is a comparative FIG. showing the refractory damage index R,; ofthe furnace wall.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1 and 2,reference numeral 1 denotes a furnace; 2, a center electrode; and 3,side electrodes. Secondary conductors, supplying current to theelectrodes, are shown in form of elements 2 and 3, electrode supportingarms; and 4 and 5, masts for raise and lowering these arms and therebythe electrodes 2 and 3. The electrodes are so arranged that the tipsthereof are spaced apart from each other along a circle having adiameter D by a relatively long distance, but their spacing is graduallyreduced as they are moved downwardly and are finally arranged along acircle having a diameter of d. In the first embodiment shown in FIG. 1,the circles D and d are coaxial so that the paths of the electrodes formthe respective sides of an inverted frustum of triangular pyramid. Thesame is true in case of the second embodiment shown in FIG. 2, but thecircle d is not coaxial with the circle D.

FIGS. 3 and 4 illustrate the embodiments in which arm 2', as shown, aswell as the length of the center electrode 2 is longer than those of theside electrodes 3. However, the paths of the movements of theseelectrodes 2 and 3 correspond to' those of the embodiments shown inFIGS. 1 and 2 respectively.

All of the arrangements described above serve to maintain thethree-phase powers in equilibrium or balanced state as describedhereinbefore. The arrows shown in FIGS. 1 and 2 indicate the downwarddirections of the electrodes,

Referring to FIG. 5 illustrating the essential parts of the embodimentshown in FIG. 1, like numerals are used to designate like parts.Reference numeral 6 designates a roof; 7, the scrap or charge notmelted; and 8, a flat bath of melted charge. Through suitable guides,including apertures in roof 6, oblique electrodes 2 and 3 are loweredalong their axes to bring their tips from the circle D at the upperportion of the furnace 1 toward the circle d in closely spaced apartrelation with the level of the flat bath 8 along the paths indicated bythe dotted lines.

At the initial stage of melting, the tips of the electrodes 2 and 3 arearranged along the upper circle D so that they are spaced apart fromeach other by a relatively longer distance. Thus, the circuit impedanceof the secondary conductor circuit is relatively high so that the arccan be stabilized and the flicker phenomenon can be prevented wherebythe scrap or charge 7 may be rapidly and uniformly melted. As themelting operation is advanced, the spacing between the tips of theelectrodes is gradually reduced as the electrodes are lowered.

When all of the scrap or charge 7 is melted into the flat bath 8, thetips of the electrodes 2 and 3 are positioned along the smaller circle dso that the impedance of the secondary conductor circuit becomes lower.Thus, the heat generated by the arcs can be concentrated at the centerportion of the flat bath 8 so that the efficiency of transmitting theheat from the arcs to the molten metal can be remarkably improved,thereby preventing the hot spot phenomenon of the furnace wallrefractory and permitting the molten metal temperature to increase athigh velocity.

According to the embodiments shown in FIGS. 3 and 4, the longer centerelectrode 2 may be employed without spacing apart the electrodes by alonger distance (in the flat bath state) and the electrodes may bearranged to form the sides of an inverted frustum of triangular pyramid.Thus, the unbalanced impedance of the three-phase circuit and thehotspot phenomenon on the elevating masts may be advantageouslyeliminated.

For the better understanding of the presenfinve ntiori, one Example willbe described hereinafter.

An arc furnace having the nominal capacity of 70 tons, the shelldiameter of 5,800 mm., the capacity of the transformer of 42,000 kva.and the electrode diameter of 20 inches was used. The electrodes werearranged as in the case of the embodiment shown in FIG. 1. The angle ofinclination of each electrode was 5. The results of the analyses basedupon the circuit impedance short circuit tests at the upper limitposition of the electrodes, at the position mm. above the level of theflat bath and at the lower limit position of the electrodes, were asshown in Table l.

TABLE 1.-COM1A RISON OF CIRCUIT IMPEDANCEEA'I THE lOSI'liON ()I THE 1 1it will be readily seen that the difference in the circuit imi pedancesat the upper and lower limit positions of the electrodes reaches 20percent so that the circuit impedance is selfcontrolled depending uponthe melting operation. According to this Example, it is expected thatthe melting time may be reduced by percent and the refining time, by IDpercent.

The pitch circle diameter (p.c.d.) of the tips of the electrodes in theflat bath state becomes 870 mm. in diameter so 3 that it is expected theservice life of the refractory of the fur- X 10 nace wall will beincreased by approximately 25 percent. (See FIG. 6).

In the higher power three-phase AC arc furnaces it is required that thesecondary circuit impedance must be reduced as much as possible in orderto improve the high enerl5 gy arc heat transmission efficiency and toreduce the power loss in the secondary conductor circuit. On the otherhand, in case of the operation with a high current in a low impedancecircuit, the arcs are not stabilized at the initiation of melting whilethe flicker phenomenon occurs. From the foregoing, it

will be seen that the present invention has succeeded to solve these twocontradicting fundamental problems and to eliminate the hot spotphenomenon of the furnace wall refractory. Without any additionalinstallation or equipment and control, the following advantages accruefrom the present invention.

i. At the initial stage, the arcs may be stabilized because of arelatively higher furnace circuit impedance while the flicker phenomenoncan be prevented; and

ii. When the charge is melted so that the molten bath is 'formed and theconditions for stabilizing the arcs (even with the tips of the electrodelowered) are satisfied, the

low impedance and high power operation may be carried out so that theheat produced by the arcs may be concentrated at the center of themolten bath thereby eliminating the hot spot phenomenon of the furnacewall refracto- The present invention, accordingly permits the extremelyeffective use of the high energy are, thereby melting the vol of thetint bath l.l5X 10""ll l 0 isidcs phase 40 velocity. Especially the highpower are furnaces may be operated with remarkably increased efficiency.

We claim:

steps of obliquely arranging the furnace electrodes; obliquely loweringthem into the furnace charge during the melting of this charge tooperate the furnace at relatively high but gradually decreasingimpedances during this melting; and thereafter operating the furnace atrelatively low impedance, established with the aid of this lowering ofthe electrodes.

2. A method according to claim 1 wherein the obliquity of the electrodearrangement and of the lowering motion is 5 from the vertical.

3. A method according to claim 1 wherein the difference Lowerlimitposition of the electronics Central phnso Z:;0.l7(i (l0- SlZ=(0.02s3-l-10.50

Z lJliXitHll Z=3.2l)(l0' ll 20 percent.

Furnace transformer:

Circular generation Average" r charge rapidly and increasing thetemperature at high 1. A heating method for direct arc furnaces,comprising the between the relatively high and low impedances ranges upto

